android - Why not I can execute ".startAnimation" in a "for" loop for several buttons? -

i have grid 12 buttons , want animate buttons. have them in vector , when intend execute ".startanimation" have "nullpointer" exception , don't know why.

i have this:

final private int tamgrid=12;   private button[] botones = new button[tamgrid];   @override protected void oncreate(bundle savedinstancestate) {     super.oncreate(savedinstancestate);     setcontentview(r.layout.activity_juego_grid12);      loopanimation=animationutils.loadanimation(this, r.anim.animacionbotongrid12);      //asociamos los elementos de la vista:     asociateelements();        }  public void asociateelements(){     string buttonid;     int resid;      for(int i=0; i<tamgrid; i++) {         buttonid="boton"+integer.tostring(i);         resid = getresources().getidentifier(buttonid, "id","butterflydevs.brainstudio");         buttons[i]=(button)findviewbyid(resid);         buttons[i].startanimation(loopanimation);     }  } 

why don't work this? error is

caused by: java.lang.nullpointerexception 

in line of startanimation.

i've noticed happens when try this:

botones[0].setonclicklistener(                  new view.onclicklistener() {                     @override                     public void onclick(view v) {                         //acciones del botón:                         botones[0].setbackgroundcolor(color.red);                      }                 }         ); 

when use index in "botones[n]". has happened anyone?

i suggest put loop inside onstart() callback. maybe buttons have not yet been created @ point in code.